1. Field of the Invention
The present invention relates generally to stepping aligners, and more particularly to a stepping aligner of the type having a stage movable to displace an object stepwise relative to an exposing position for exposing a plurality of portions of the object in succession to exposure light. The stepping aligner of this invention can be used in various fields of art for exposing various objects, for example, in a pattern lithography performed in the manufacture of electronic components such as semiconductor devices.
2. Description of the Prior Art
Conventionally, an object to be exposed is moved relative to an exposing position so that a plurality of portions of the object are exposed successively. For example, in the case of semiconductor devices, a plurality of portions of an object (wafer) to be exposed are arranged in rows in the X-direction and columns in the Y-direction. For exposing the object, one of these portions is aligned with an exposing position and then exposed to a shot of exposure light. Subsequently, the object is moved relative to the exposing position until the next portion is aligned with the exposing position. The next portion thus aligned is subsequently exposed to the exposure light. The foregoing cycle is repeated to expose the portions of the object in succession. In the specification and claims, such a relative movement between the object and the exposing position, which is performed stepwise, is referred to as a "stepping movement." A typical example of the prior art is disclosed in Japanese Patent Laid-open Publication No. 62-102523. When the exposure process is performed using a mask larger in size than a pattern to be formed, reduction projection alignment is needed. Such a reduction projection alignment is performed by a reduction projection aligner which is generally called as a "stepper." The term "stepping aligner" as used herein is, however, intended to include all the stepping aligners, such as reduction projection aligners, 1:1 projection aligners, etc., regardless of the magnification of a projection optics system used.
FIG. 5 shows the general construction of the reduction projection aligner. In this figure, numeral 6 is exposure light, 10 is an object to be exposed to the exposure light, 31 is an X-stage, 32 is a Y-stage, 41 is a ball screw for moving the X-stage 32 in the X-direction indicated by the arrow 51, and 42 is a ball screw for moving the Y-stage in the Y-direction indicated by the arrow 52.
There has been an increasing tendency to promote microminiaturization and integration of patterns to be formed on an object using the stepping aligner. In the manufacture of semiconductor integrated circuits, the minimum workable dimension of circuit patterns has been reduced to half-micron or even to the level of 0.35 .mu.m. For a semiconductor device (64MDRAM, for example) having a pattern size of less than half-micron, the pattern exposure process must be performed with a very high alignment accuracy. In practice, the necessary overlay accuracy is on the order of 0.1 .mu.m.
In the exposure technology requiring a very high precision alignment, the alignment accuracy can be deteriorated even by a trifling matter. For instance, in a KrF excimer laser stepper which is one of the effective apparatus for the high precision micromachining, the alignment accuracy in each of the X-direction and the Y-direction is not constant but varies with the individual objects (wafers) to be exposed. In addition, the dispersion of alignment accuracy in the X-direction has a different tendency from the dispersion of the alignment accuracy in the Y-direction.
FIGS. 8(A) and 8(B) are graphs showing the relationship between the alignment accuracy and the number of objects (wafers) to be exposed. FIG. 8(A) illustrates the data taken with respect to the X-direction, while FIG. 8(B) shows the data taken with respect to the Y-direction. As is apparent from the comparison between FIGS. 8(A) and 8(B), the alignment accuracy in the X-direction has a greater dispersion than the alignment accuracy in the Y-direction. In addition, FIG. 8(A) indicates that the alignment accuracy in the X-direction increases with an increase in number of the objects (wafers) and hence has a regular tendency. On the other hand, the alignment accuracy in the Y-direction varies at random regardless of the number of objects (wafers) as shown in FIG. 8(B) and, hence, has no regular tendency.
In the conventional stepping aligner, the stepping movement in the X and Y directions is generally performed in the pattern B shown in FIG. 3. More specifically, the object to be exposed is moved stepwise in the X-direction to expose a row of portions of the object in succession. Then, the object is moved in the Y-direction by a distance equal to the width of the row of portions of the object, and a next row of portions of the object are exposed successively in timed relation to the stepwise movement of the object in the X-direction. This stepping pattern B is adopted in order to improve the throughput of the exposure process. The data of the alignment accuracy shown in FIGS. 8(A) and 8(B) are taken when the exposure process is performed using the stepping pattern B shown in FIG. 3. In general, stepping of an object 10 to be exposed is carried out by using a stage 3 driven by means of an X axis ball screw 41 and a Y axis ball screw 42, as shown in FIG. 4. When the stepping is performed using the conventional stepping pattern B shown in FIG. 3, the amount of stepping movement (stepping length) in the X-direction is greater than that of the Y-direction.
It seems provably that the difference in dispersion distribution tendency between the alignment accuracy in the X-direction and the alignment accuracy in the Y-direction is caused by the stepping movement made in the pattern B shown in FIG. 3. Since the amount of stepping movement in the X-direction is greater than the amount of stepping movement in the Y-direction, the dispersion of the alignment accuracy becomes greater in the X-direction than in the Y-direction. In addition, during the stepping movement, movable components generate heat due to mechanical factors such as friction acting between the movable components. The heat thus generated seems to have direct effects upon the dispersion of alignment accuracy. Consequently, the stepping movement in the X-direction, which is greater in amount than that of the Y-direction, generates a greater amount of heat than the stepping movement in the Y-direction. In addition to the heat generated during the stepping movement of the stage in the X and Y directions, heat is also generated during shot (exposure of a portion of the object with exposure light) in the exposure process. In the stepping aligners, such as excimer laser steppers of the type wherein the TTL (through the lens) alignment system cannot be used, an alignment base line may be displaced due to the heat generated during the exposure process.
In order to perform a high precision alignment to obtain the above-mentioned minimum workable dimension of circuit patterns, the aligner must has a high precision X-Y-.theta.-Z stage. Conventionally, the X-Y-.theta.-Z stage is positioned by using interferometers. However, since the interferometers are susceptible to heat, the alignment accuracy tends to decrease when the interferometers are subjected to heat. FIG. 6 illustrates a manner in which the X-Y-.theta.-Z stage is positioned by a pair of interferometers 81, 82. A laser beam emitted from each of the interferometers 81, 82 returns to the interferometer 81, 82 after it is reflected by a corresponding one of two bar mirrors la, lb disposed on the stage. In this instance, the positioning accuracy is determined by the fluctuation of temperature of air surrounding the path of the laser beam, and the flatness of the bar mirrors la, lb. On the other hand, when the exposure process performed in the stepping aligner for forming patterns on a single object (wafer), the stage is moved a plurality of times equal to the number of shots. While the stage is moving, heat is generated from movable parts. A study made by the present inventor indicated that the heat generated during the movement of the stage raises the temperature by about 1.degree. C. at maximum. When the heat is transmitted to the bar mirrors la, lb, the bar mirrors la, lb are thermally deformed with the result that the stage is misaligned within a range of about 0.1 .mu. m. This misalignment results in a misalignment between a portion of the object (wafer) to be exposed and the exposing position where exposure light is projected. Thus, the alignment accuracy is lowered by heat.
As shown in FIG. 6, the stage is composed of an X-stage 31, a Y-stage 32, and a .theta.-Z stage 33. The X-stage 31 is movable in the direction of the arrow 51 in the same figure, while the Y-stage 32 is movable in the direction of the arrow 52. The X-stage 31 is driven by a motor 71 via a ball screw 41. The Y-stage 32 is driven by a motor 72 via a ball screw 42.